Pool type, liquid metal cooled nuclear fission reactors typically comprise a vertically positioned tank or vessel having an open upper end provided with a cover member closing off the opening in the upper end. The reactor vessel, which has no components penetrating through its wall structure, contains a core of fissionable nuclear fuel conventionally positioned centrally and adjacent to the lower end of the reactor vessel or tank. The fuel core of fissionable material is submerged within a pool of liquid metal coolant, such as sodium, which substantially fills the reactor vessel to a level sufficient to submerge the fuel units both within the core and handled at a level above the fuel core. Liquid metal coolant is circulated through the fuel core to remove fission produced heat from the core and transfer the heat removed from the core to a heat exchanger for conveyance and use outside of the reactor vessel by means of a fluid pumping system. Operating means comprising fission control rods are suspended down from the reactor vessel top member for reciprocal movement into and out from the fuel core.
Typically the area or plenum within the reactor vessel above the surface of the pool of liquid metal is filled with an inert gas such as nitrogen or argon to preclude reactive material from contacting the coolant.
Heat generating, fissionable fuel comprising uranium, plutonium and/or thorium metal, or alloys thereof, in the form of small pellets or slugs, is sealed within metal, such as stainless steel, tubes or elongated containers. A plurality of such fuel containing tubes are combined into a single unit or assembly. Typically these reactor fuel core assemblies are of hexagonal or similar angular cross sections, and their lower end is provided with a conical end member to facilitate seating in a bottom support structure. The upper end of the reactor fuel core assembles is provided with a top member having a cylindrical portion of substantially smaller diameter than the assembled body of joined fuel tubes which projects axially upward from the assembly, and an end cap thereon comprising a transverse annular flange of hexagonal or similar multi angular cross section. This configuration of the top member, which resembles the head end of a lag bolt, provides for vertical aligning of the fuel assembly in both core and storage mounting brackets or supports.
An in-vessel transfer device extends down from a circular revolving section of the reactor vessel cover member whereby it can move over a substantial cross-sectional area of the central portion of the reactor vessel including the fuel core. The in-vessel transfer device serves to introduce and remove fissionable fuel in the form of the typical assembled bodies into and out from the reactor vessel, and within and about the reactor vessel including to and from the fuel core and internal reactor vessel storage means such as racks. In-vessel transfer devices conventionally comprise a vertical support structure which extends through the sealing reactor vessel top member whereby it can be manipulated by an operator from above the reactor vessel. The vertical support structure is provided with a gripping mechanism for securely attaching to and holding assembled fuel bodies whereby they can be safely and effectively moved about within the reactor vessel remotely by an operator located a distance above the reactor vessel.
A conventional type of gripping mechanism mounted on the vertical support structure as a means for extending the operating scope or reach and maneuverability is a device having a panographic action or system. Such a means reciprocally mounted on the vertical support structure provides significantly extended vertical and lateral maneuverability and grasping action for fuel assemblies within the reactor vessel by an externally located operator from a distance above the reactor vessel.
Although various grasping systems can be employed for attaching to the fuel assemblies, a preferred means comprises a socket type of union which minimizes the potential for causing damage to the fuel assemblies. A common and suitable socket union for securing fuel assemblies consists of a socket cavity in the uppermost end of the fuel assembly units and a counterpart mating socket extension having retractable lateral projections affixed to the gripping mechanism. Thus, the socket extension can vertically enter down into a mating socket cavity of a fuel assembly with the lateral projections retracted, and upon extension of the lateral projections of the socket while mated within the socket cavity into lateral counterpart receiving recesses, the gripping mechanism will be effectively locked to the fuel assembly for secure transfer within the vessel.
A transfer basket or receptacle is conventionally employed with the in-vessel transfer device for the conveyance of fuel core assemblies into and out from the reactor vessel of such pool type, liquid metal cooled nuclear reactors. The typical transfer basket comprises an open top container or receptacle suspended from a support member extending down through the reactor vessel top member and into the vessel interior. The basket member generally consists of an elongated container or receptacle of a size adequate to accept at least one fuel core assembly through an open upper end or top and essentially enclose the assembly within its internal cavity whereby the fuel core assemblies can be effectively retained and securely moved or transferred into and out from the reactor vessel without the possibility of accidental disgorgement.
However, the conventional transfer baskets being top loading, the in-vessel transfer device in handling the elongated fuel core assemblies between the fuel core or in-vessel storage racks, must exercise extensive vertical movement in raising the fuel assemblies up above the transfer basket for their introduction or withdrawal through top openings of the transfer baskets.